(1) Field of the Invention
This invention relates to p-n cadmium telluride-cadmium sulfide photovoltaic cells, particularly of the thin-film type.
(2) State of the Prior Art
Effective conversion from expensive petroleum-based energy sources to solar energy sources, such as photovoltaic cells, has been delayed prior to this invention by two factors--the cost of mass-producing such cells and the low conversion efficiency achieved by such cells. Any improvement in either factor can move industry towards the use of more solar cells, and an improvement in both has been a long-sought goal.
A study by R. Moore reported in Solar Energy, 18, p. 225 (1976) indicates that thin-film inorganic semiconductor photovoltaic cells should be able to meet the two-fold goal noted above if the individual semiconductive layers do not exceed 5 microns in thickness, assuming conversion efficiencies of at least 5%. Implied is the conclusion that such cells would be even more useful if the conversion efficiencies were higher. The study concludes with the statement that such thin-film cells as described are predicated on "a currently non-existent technological base."
Conversion efficiencies of 5% or more have been difficult to achieve in thin-film solar cells, particularly in solar cells fabricated from CdS and CdTe, hereinafter identified as CdS/CdTe solar cells. Such CdS/CdTe cells have a distinct cost advantage, compared for example to single-crystal silicon cells. Conversion efficiencies, however, must be increased to render them useful. Therefore, considerable effort has been expended to establish techniques for manufacturing such cells with improved conversion efficiencies, as well as reduced thicknesses. Vapor deposition or similar vapor-phase formation of either CdTe upon a crystal of CdS, or of CdS upon a crystal of CdTe, has been used as a means of partially reducing the film thickness, enhancing conversion efficiency, or both, as is reported in articles by A. Fahrenbruch et al, Appl. Phys. Letters, 25, p. 605 (1974); R. Bube et al, Report NSF/RANN/Se/AER-75-1679/76/1; and K. Yamaguchi et al, Japan J. Appl. Phys., 15, p. 1575 (1976). Such prior techniques have relied largely upon the use of single crystal substrates for the vapor deposition. Utilization of single crystal substrates, however, precludes the achievement of the economic advantages associated with thin-film cells, since a single crystal must be grown and sliced. The slicing operation necessarily involves waste of material and produces cells which at best are 50 to 100 microns thick. Thus, it is not possible to manufacture a bilayer solar cell, one layer of which is a single crystal, that is as thin as 10 microns as suggested in the Moore study. In each of the foregoing, the vapor deposition or similar process was done in the absence of oxygen.
In Bonnet, "New Results on the Development of a Thin-Film p-CdTe-n-CdS Heterojunction Solar Cell", 9th IEEE Photovoltaic Specialist Conference, p. 129 (1972), there is reported a thin-film cell of polycrystalline material alleged to have produced a conversion efficiency of 5%. However, the reported I.sub.sc value, measured with 50 mW/cm.sup.2 illumination, was 1.5 mA per 10 mm.sup.2, or 15 mA/cm.sup.2. This corresponds to an I.sub.sc value of 30 mA/cm.sup.2 for a 100 mW/cm.sup.2 illumination, the standard "AM1" sunlight condition. Since the theoretical limit of I.sub.sc under AM1 conditions is only about 24 mA/cm.sup.2, clearly there was a positive error in the observations and the conversion efficiency was less than the reported value of 5%.